[PATCH 1/3] drm/shmem-helper: Import dmabuf without mapping its sg_table

Christian König christian.koenig at amd.com
Mon May 5 14:32:10 UTC 2025


Hi Shixiong,

for drm changes please base your patches on drm-misc-next or drm-next.

That is probably fixed by this one here:

commit b72f66f22c0e39ae6684c43fead774c13db24e73
Author: Christian König <christian.koenig at amd.com>
Date:   Tue Feb 11 17:20:53 2025 +0100

    dma-buf: drop caching of sg_tables
    
    That was purely for the transition from static to dynamic dma-buf
    handling and can be removed again now.
    
    Signed-off-by: Christian König <christian.koenig at amd.com>
    Reviewed-by: Simona Vetter <simona.vetter at ffwll.ch>
    Reviewed-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20250211163109.12200-5-christian.koenig@amd.com

After this patch SG tables where only created when necessary.

Regards,
Christian.

On 5/5/25 16:22, oushixiong wrote:
> Hi Christian,
> 
> My patch is based on linux-next, so this patch is not based on the latest code. Then, I'd like to ask which patch resolved the issue with sg-tables?
> 
> 
> Thanks and Regards,
> 
> Shixiong.
> 
> 
> 在 2025/5/5 19:25, Christian König 写道:
>> Hi Thomas & Shixiong,
>>
>> first of all the patch is still based on outdated code. For example the cache_sgt_mapping member is already removed in drm-misc-next.
>>
>> So if I'm not completely mistaken the issue is already resolved upstream.
>>
>> Regards,
>> Christian.
>>
>> On 5/5/25 13:12, Thomas Zimmermann wrote:
>>> (cc'ing Christian)
>>>
>>> Hi,
>>>
>>> I don't feel qualified to fully review this patch.
>>>
>>> It would be good to have the issue with sg-tables solved, but I dislike the dedicated initializer macros. So my question is if this has any drawbacks. Or could we make this available and the default for all shmem-based drivers?
>>>
>>> Best regards
>>> Thomas
>>>
>>> Am 01.05.25 um 08:43 schrieb oushixiong1025 at 163.com:
>>>> From: Shixiong Ou <oushixiong at kylinos.cn>
>>>>
>>>> [WHY]
>>>> 1. Drivers using DRM_GEM_SHADOW_PLANE_HELPER_FUNCS and
>>>>      DRM_GEM_SHMEM_DRIVER_OPS (e.g., udl, ast) do not require
>>>>      sg_table import.
>>>>      They only need dma_buf_vmap() to access the shared buffer's
>>>>      kernel virtual address.
>>>>
>>>> 2. On certain Aspeed-based boards, a dma_mask of 0xffff_ffff may
>>>>      trigger SWIOTLB during dmabuf import. However, IO_TLB_SEGSIZE
>>>>      restricts the maximum DMA streaming mapping memory, resulting in
>>>>      errors like:
>>>>
>>>>      ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 bytes), total 32768 (slots), used 0 (slots)
>>>>
>>>> [HOW]
>>>> Provide a gem_prime_import implementation without sg_table mapping
>>>> to avoid issues (e.g., "swiotlb buffer is full"). Drivers that do not
>>>> require sg_table can adopt this.
>>>>
>>>> Signed-off-by: Shixiong Ou <oushixiong at kylinos.cn>
>>>> ---
>>>>    drivers/gpu/drm/drm_gem_shmem_helper.c | 95 ++++++++++++++++++++++++++
>>>>    include/drm/drm_gem_shmem_helper.h     | 24 +++++++
>>>>    2 files changed, 119 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> index d99dee67353a..9e41e350ff6f 100644
>>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> @@ -39,6 +39,7 @@ MODULE_IMPORT_NS("DMA_BUF");
>>>>    static const struct drm_gem_object_funcs drm_gem_shmem_funcs = {
>>>>        .free = drm_gem_shmem_object_free,
>>>>        .print_info = drm_gem_shmem_object_print_info,
>>>> +    .export = drm_gem_shmem_object_prime_export,
>>>>        .pin = drm_gem_shmem_object_pin,
>>>>        .unpin = drm_gem_shmem_object_unpin,
>>>>        .get_sg_table = drm_gem_shmem_object_get_sg_table,
>>>> @@ -799,6 +800,100 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_sg_table);
>>>>    +const struct dma_buf_ops drm_gem_shmem_prime_dmabuf_ops =  {
>>>> +    .cache_sgt_mapping = true,
>>>> +    .attach = drm_gem_map_attach,
>>>> +    .detach = drm_gem_map_detach,
>>>> +    .map_dma_buf = drm_gem_map_dma_buf,
>>>> +    .unmap_dma_buf = drm_gem_unmap_dma_buf,
>>>> +    .release = drm_gem_dmabuf_release,
>>>> +    .mmap = drm_gem_dmabuf_mmap,
>>>> +    .vmap = drm_gem_dmabuf_vmap,
>>>> +    .vunmap = drm_gem_dmabuf_vunmap,
>>>> +};
>>>> +
>>>> +/**
>>>> + * drm_gem_shmem_prime_export - implementation of the export callback
>>>> + * @shmem: shmem GEM object
>>>> + */
>>>> +struct dma_buf *drm_gem_shmem_prime_export(struct drm_gem_shmem_object *shmem,
>>>> +                       int flags)
>>>> +{
>>>> +    struct drm_gem_object *obj = &shmem->base;
>>>> +    struct drm_device *dev = obj->dev;
>>>> +    struct dma_buf_export_info exp_info = {
>>>> +        .exp_name = KBUILD_MODNAME, /* white lie for debug */
>>>> +        .owner = dev->driver->fops->owner,
>>>> +        .ops = &drm_gem_shmem_prime_dmabuf_ops,
>>>> +        .size = obj->size,
>>>> +        .flags = flags,
>>>> +        .priv = obj,
>>>> +        .resv = obj->resv,
>>>> +    };
>>>> +
>>>> +    return drm_gem_dmabuf_export(dev, &exp_info);
>>>> +}
>>>> +
>>>> +/**
>>>> + * drm_gem_shmem_prime_import - Import dmabuf without mapping its sg_table
>>>> + * @dev: Device to import into
>>>> + * @dma_buf: dma-buf object to import
>>>> + *
>>>> + * Drivers that use the shmem helpers but also wants to import dmabuf without
>>>> + * mapping its sg_table can use this as their &drm_driver.gem_prime_import
>>>> + * implementation.
>>>> + */
>>>> +struct drm_gem_object *drm_gem_shmem_prime_import(struct drm_device *dev,
>>>> +                          struct dma_buf *dma_buf)
>>>> +{
>>>> +    struct dma_buf_attachment *attach;
>>>> +    struct drm_gem_shmem_object *shmem;
>>>> +    size_t size;
>>>> +    int ret;
>>>> +
>>>> +    if (dma_buf->ops == &drm_gem_shmem_prime_dmabuf_ops) {
>>>> +        struct drm_gem_object *obj;
>>>> +
>>>> +        obj = dma_buf->priv;
>>>> +        if (obj->dev == dev) {
>>>> +            /*
>>>> +             * Importing dmabuf exported from our own gem increases
>>>> +             * refcount on gem itself instead of f_count of dmabuf.
>>>> +             */
>>>> +            drm_gem_object_get(obj);
>>>> +            return obj;
>>>> +        }
>>>> +    }
>>>> +
>>>> +    attach = dma_buf_attach(dma_buf, dev->dev);
>>>> +    if (IS_ERR(attach))
>>>> +        return ERR_CAST(attach);
>>>> +
>>>> +    get_dma_buf(dma_buf);
>>>> +
>>>> +    size = PAGE_ALIGN(attach->dmabuf->size);
>>>> +
>>>> +    shmem = __drm_gem_shmem_create(dev, size, true, NULL);
>>>> +    if (IS_ERR(shmem)) {
>>>> +        ret = PTR_ERR(shmem);
>>>> +        goto fail_detach;
>>>> +    }
>>>> +
>>>> +    drm_dbg_prime(dev, "size = %zu\n", size);
>>>> +
>>>> +    shmem->base.import_attach = attach;
>>>> +    shmem->base.resv = dma_buf->resv;
>>>> +
>>>> +    return &shmem->base;
>>>> +
>>>> +fail_detach:
>>>> +    dma_buf_detach(dma_buf, attach);
>>>> +    dma_buf_put(dma_buf);
>>>> +
>>>> +    return ERR_PTR(ret);
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import);
>>>> +
>>>>    MODULE_DESCRIPTION("DRM SHMEM memory-management helpers");
>>>>    MODULE_IMPORT_NS("DMA_BUF");
>>>>    MODULE_LICENSE("GPL v2");
>>>> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
>>>> index cef5a6b5a4d6..78ef91593a8e 100644
>>>> --- a/include/drm/drm_gem_shmem_helper.h
>>>> +++ b/include/drm/drm_gem_shmem_helper.h
>>>> @@ -110,6 +110,8 @@ int drm_gem_shmem_vmap(struct drm_gem_shmem_object *shmem,
>>>>    void drm_gem_shmem_vunmap(struct drm_gem_shmem_object *shmem,
>>>>                  struct iosys_map *map);
>>>>    int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct *vma);
>>>> +struct dma_buf *drm_gem_shmem_prime_export(struct drm_gem_shmem_object *shmem,
>>>> +                          int flags);
>>>>      int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem);
>>>>    void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem);
>>>> @@ -168,6 +170,18 @@ static inline void drm_gem_shmem_object_print_info(struct drm_printer *p, unsign
>>>>        drm_gem_shmem_print_info(shmem, p, indent);
>>>>    }
>>>>    +/**
>>>> + * drm_gem_shmem_object_prime_export - GEM object function for export()
>>>> + * @obj: GEM object
>>>> + *
>>>> + */
>>>> +static inline struct dma_buf *drm_gem_shmem_object_prime_export(struct drm_gem_object *obj,
>>>> +                                int flags)
>>>> +{
>>>> +    struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>>>> +
>>>> +    return drm_gem_shmem_prime_export(shmem, flags);
>>>> +}
>>>>    /**
>>>>     * drm_gem_shmem_object_pin - GEM object function for drm_gem_shmem_pin()
>>>>     * @obj: GEM object
>>>> @@ -276,6 +290,8 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
>>>>                        struct sg_table *sgt);
>>>>    int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
>>>>                      struct drm_mode_create_dumb *args);
>>>> +struct drm_gem_object *drm_gem_shmem_prime_import(struct drm_device *dev,
>>>> +                          struct dma_buf *buf);
>>>>      /**
>>>>     * DRM_GEM_SHMEM_DRIVER_OPS - Default shmem GEM operations
>>>> @@ -287,4 +303,12 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
>>>>        .gem_prime_import_sg_table = drm_gem_shmem_prime_import_sg_table, \
>>>>        .dumb_create           = drm_gem_shmem_dumb_create
>>>>    +/**
>>>> + * This macro provides a shmem GEM operations that implementate a simple
>>>> + * gem_prime_import.
>>>> + */
>>>> +#define DRM_GEM_SHMEM_SIMPLE_DRIVER_OPS \
>>>> +    .gem_prime_import    = drm_gem_shmem_prime_import, \
>>>> +    .dumb_create        = drm_gem_shmem_dumb_create
>>>> +
>>>>    #endif /* __DRM_GEM_SHMEM_HELPER_H__ */
> 



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